Nitrogen oxides (NOx) generally refer to a variety of compounds comprising oxygen and nitrogen, such as nitrogen monoxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O), dinitrogen tetroxide (N2O4), dinitrogen pentoxide (N2O5), etc. In particular, NO and NO2, which account for most of nitrogen oxides in exhaust gas, are not only the main cause for acid rain along with sulfur oxides (SOx), but are air pollutants that induce photochemical smog.
Methods for removing such nitrogen oxides can be largely divided into a wet method and a dry method. Wet methods are advantageous in removing both nitrogen oxides and sulfur oxides and are generally used in processes where nitrogen oxide is produced in a small amount. However, in the wet process, because NO has low water solubility, it should be oxidized to NO2 preceding absorption in the liquid solution. Such a conversion process is problematic since it involves high costs and may produce N2O3 and N2O4 as by-products resulting in potential water pollution.
The major dry processes for nitrogen oxides are the selective non-catalytic reduction (SNCR) method and the selective catalytic reduction (SCR) method. The SNCR method selectively reduces nitrogen oxides to nitrogen and water by injecting only ammonia at a high temperature between 850 and 1050° C. in the absence of a catalyst. The SCR method involves the use of gaseous ammonia as a reducing agent and a catalyst to reduce nitrogen oxides to nitrogen and water at a relatively low temperature (150-450° C.).
The SNCR method has the advantage of removing more than 50% of nitrogen oxides at a relatively low cost, but emitted, unreacted ammonia forms ammonium salts, which may clog up the end part of the reactor or cause corrosion. Further, due to the narrow operation temperature range, there are difficulties in commercializing the process. As a result, the SCR method is now being regarded as the most advanced, safest, and most cost effective technology for treating nitrogen oxides. This method has the advantage of having a nitrogen oxide removal ratio of 90% or higher and does not require any additional post-treatment processes. In SCR processes, catalyst performance is the key factor. Thus, various types of catalysts for SCR processes have been suggested, ranging from precious metal catalysts to base metal catalysts. Of the catalysts developed so far, vanadium-based catalysts using titanium oxide (TiO2) as a support have been most widely used in actual processing. A SCR process using a vanadium-based catalyst employing TiO2 as a support is a very efficient technology for converting NOx to N2 at a temperature of around 350° C. However, because the catalyst can be worn, exchanged, and toxic, the process has the problem of decreased NOx conversion ratios or side-reactions, such as oxidization of reducing agents prior to the removal reaction. Furthermore, in SCR processes, gaseous ammonia may trigger oxidization to lower the catalyst performance when the reaction temperature is above 450° C., or nitrogen oxides may be regenerated from the gaseous ammonia. If the reaction temperature is less than 150° C., the gaseous ammonia may react with moisture in the exhaust gas to form ammonium nitrate or ammonium sulfate, thereby hampering the process.
As a result, there is a need for a simple nitrogen oxide treatment method with high efficiency that can resolve the above-mentioned problems in the SCR process which uses gaseous ammonia as a reducing agent.